Intercoat adhesion : How can intercoat adhesion be studied and what influences intercoat adhesion

Sammanfattning: Coatings cover most objects in our daily lives. They can have functional properties such as protecting the coated material or esthetic properties to decorate and provide pleasant appearance. Coatings have historically dried by evaporation of solvents, but now, UV curing coatings are on the uprising. Acrylate functional groups on polymers, oligomers and monomers react through radical chain polymerization initiated by photoinitiators. Photoinitiators are molecules which forms radicals when irradiated with UV light.Coatings are often applied on multiple layers, each layer providing one function for the overall system. To function properly and have desired life time the layers must adhere to each other. The problem of intercoat adhesion has yet to find good techniques for analysis and explanation of what influences it. Many theories have been suggested and most likely many play a part in the overall adhesion.The master thesis project aim to investigate which properties that influences intercoat adhesion and how it can be studied. To relate properties of coatings to the intercoat adhesion one UV curing primer is set to use for all coating systems, and various topcoats have been produced with slight changes in formulation. The study can be divided into two parts; a pre-study and a main study. The pre-study follows up on a previous master thesis conducted at Sherwin-Williams AB. The pre-study has waterbased and waterbased UV-curable topcoats. The main study has UV curable topcoats. Properties which are studied in the main study are chemical backbone of the binder, functionality of monomers, influence of addition of wetting agents, defoamers, fillers and pigment, viscosity, density, pH, curing degree at depth, surface energy, surface tension, surface polarity and monomer to binder ratio.The pull-off method is the best method of analysis of intercoat adhesion today. A dolly is glued to a surface and then lifted, the force of lifting the dolly is measured. The method has one significant drawback; the break must be completely in the interphase of where the adhesion wishes to be analyzed. Throughout this study most interphase failures were between substrate and primer, resulting in no value for intercoat adhesion.No correlation was found between intercoat adhesion and chemical backbone of the binder, functionality of monomers, addition of wetting agents, defoamers, viscosity, pH, surface energy, surface tension or surface polarity. Lower intercoat adhesion was observed for coatings containing talc, calcium carbonate and titanium dioxide. Coating containing titanium dioxide showed insufficient curing above a coating thickness of 40 μm. The insufficient curing could be observed as wrinkles on the surface and liquid coating remaining in the coating interphase. Curing degree in depth of the topcoat is believed to be the main reason to decreased intercoat adhesion for the coating containing titanium dioxide. The insufficient curing could not be confirmed with infrared spectrometry. The reason why talc and calcium carbonate showed decreased intercoat adhesion is not known. Indications suggest that a lower monomer to binder ratio decrease intercoat adhesion, theories to explain this are the high viscosity and the low number of functional groups per volume. A higher number of functional groups per volume could increase the number of crosslinks formed between topcoat and primer.